Electroluminescent device

ABSTRACT

Broadly speaking, embodiments of the present invention provide a solid state light-emitting device and a method of manufacturing the solid state light-emitting device. The method comprises preparing a thin layer of semiconducting perovskite nanoparticles embedded in a matrix or blend of a material that has a wider band gap than the semiconducting perovskite nanoparticles. In embodiments, the method comprises blending a solution of a semiconducting perovskite material or a precursor therefor with a solution of a material that has a wider band gap than the semiconducting perovskite material or a precursor therefor followed by removal of the solvent from the mixture thus formed, to give the semiconducting perovskite nanoparticles embedded in a matrix or blend of the material that has a wider band gap than the semiconducting perovskite nanoparticles.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/531,404, filed on May 26, 2017, which is a U.S. National Stageapplication filed under 35 USC 371(c) of PCT/GB2015/053557, filed onNov. 20, 2015, which claims priority to Great Britain Patent ApplicationNo. 1421133.8, filed on Nov. 28, 2014. All of the above-referencedapplications are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

This invention relates to a solid state light-emitting device and inparticular to a perovskite-based light-emitting diode, aperovskite-based emissive phosphor and related fabrication methods.

BACKGROUND TO THE INVENTION

In its most basic form a light emitting diode (LED) comprises a lightemitting layer which is positioned between an anode and a cathode. Ahole injection layer may be incorporated between the anode and the lightemitting layer (also known as the active or emissive layer). Itfunctions to decrease the energy difference between the work function ofthe anode and the valence band or highest occupied molecular orbital(HOMO) of the light emitting layer, thereby increasing the number ofholes introduced into the light emitting layer. Broadly speaking, inoperation, holes are injected through the anode, and if present the holeinjection layer, into the active layer, and electrons are injected intothe active layer through the cathode. The holes and electrons combine inthe light emitting layer radiatively to provide light. Equivalently, anelectron injection layer between cathode and light-emitting layer canplay the same role in controlling the injection of electrons into thelight-emitting layer. A further role for these injection layers is toconfine carriers within the device, so that under forward electric bias,electrons injected from the cathode into the light-emitting layer aresignificantly prevented from leaving this layer via the hole-injectinglayer, and equivalently, holes injected into the light-emitting layerfrom the anode are significantly prevented from leaving this layer viathe electron-injecting layer.

Some devices also incorporate a thin polymer interlayer between the holeinjection layer and the light emitting layer. This plays an importantrole in improving the device efficiency and the lifetime of LEDs. Forexample, with an interlayer, blue light-emitting polymer organiclight-emitting diodes (LEP OLEDs) with an external quantum efficiency ofgreater than 5% can be achieved, which is 35% higher than without theinterlayer. It is believed that this may be due to the prevention ofexciton quenching at the hole injection layer/light emitting layerinterface.

Over the past two decades, solid state light-emitting devices based ondirect bandgap semiconductors have been utilized as energy efficientsources of lighting. However, the fabrication of these devices typicallyrelies on expensive high temperature and high vacuum processes, such asmolecular beam epitaxy or thermal sublimation, rendering themuneconomical for use in large area displays.

Solution processing of luminescent semiconductors presents aparticularly attractive option for the low-cost fabrication oflight-emitting devices [see Burroughes et al. Nature 347, 539-541(1990); Greenham et al, Nature 365, 628-630 (1993); Colvin et al, Nature370, 354-357 (1994); and Coe et al, Nature 420, 800-803 (2002)]. Recentwork on high-efficiency organometal halide perovskite photovoltaics hasshown these materials to possess both the remarkable qualities oftraditional semiconductors and the facile processability of organicsemiconductors [Lee et al, Science 338, 643-647,doi:10.1126/science.1228604 (2012); Burschka et al., Nature 499,316-319, doi:10.1038/nature12340 (2013); Liu et al., Nature 501,395-398, doi:10.1038/nature12509 (2013); Stranks, et al., Science 342,341-344, doi:10.1126/science.1243982 (2013); and Ball et al, Energy &Environmental Science 6, 1739-1743, doi:10.1039/c3ee40810h (2013)].Further prior art can be found in, e.g. JP 2008-227330 A.

The semiconducting perovskite materials benefit from low cost andearth-abundance, and can be deposited at low temperatures under ambientconditions. More recently, bright and colour-controlledelectroluminescence was reported in perovskite light-emitting diodes(PeLED), thereby opening up a potential range of display and lightingapplications for these materials [see Tan, Z.-K. et al., Nat Nano 9,687-692, doi:10.1038/nnano.2014.149 (2014)]. However, the quantumefficiencies in these devices remain modest due to difficulties in theformation of uniform thin films.

Light emission occurs when injected electrons and holes meet in theperovskite layer and recombine radiatively. However, it is easy forinjected charges to bypass the semiconducting perovskite layer throughpinholes in the thin films, leading to non-radiative current losses anda lower efficiency. Difficulties in the formation of uniform andpinhole-free semiconducting perovskites are well known, due to thematerial's crystalline nature. This problem is further exacerbated bythe sublimation of excess methylammonium halide precursor during thermalannealing, thereby leaving voids in the perovskite layer. An establishedtechnique to overcome this problem involves sequential or vapordeposition of the perovskite precursors [see Liu et al above and Chen,Q. et al., Journal of the American Chemical Society 136, 622-625,doi:10.1021/ja411509g (2013)], although these methods only improve filmformation and cannot completely eliminate pinholes.

There is therefore a need to provide an improved method for thepreparation of semiconducting perovskite nanoparticle films whichaddress this problem of the formation of pinholes in the films. Theprovision of such films will enable the manufacture of improved solidstate light-emitting devices and other devices, e.g., solar cells, inwhich semiconducting perovskite films can be incorporated as asemiconductor. The luminescent nature of such films also makes themuseful for emissive phosphors applications.

SUMMARY OF THE INVENTION

The applicant has found that the problems of current leakage throughperovskite films due to pinhole formation can be overcome through theembedding of semiconducting perovskite nanoparticles in a material thathas a wider band gap than the semiconducting perovskite nanoparticles.

Thus, in a first aspect of the present invention there is provided amethod for the preparation of a thin layer of semiconducting perovskitenanoparticles embedded in a matrix or blend of a material that has awider band gap than the semiconducting perovskite nanoparticles, themethod comprising blending a solution of a semiconducting perovskitematerial or a precursor therefor with a solution of a material that hasa wider band gap than the semiconducting perovskite material or aprecursor therefor followed by removal of the solvent from the mixturethus formed, to give said semiconducting perovskite nanoparticlesembedded in a matrix or blend of the material that has a wider band gapthan the semiconducting perovskite nanoparticles.

The semiconducting perovskite nanoparticles form in situ when a blend ofsemiconducting perovskite precursor and material having a wider band gapthan the semiconducting perovskite nanoparticles is deposited. Theuniformly distributed perovskite nanoparticles provide good lightemission, while the material having a wider band gap than the perovskitenanocrystals fills in the surrounding voids to block non-radiativecurrent losses.

In a second aspect of the present invention there is provided a thinlayer of semiconducting perovskite nanoparticles embedded in a matrix orblend of a material that has a wider band gap than the semiconductingperovskite nanoparticles.

In a third aspect of the present invention, there is provided a thinlayer of semiconducting perovskite nanoparticles embedded in a matrix orblend of a material that has a wider band gap than the semiconductingperovskite nanoparticles prepared according to the method according tothe first aspect of the present invention.

In a fourth aspect of the present invention, there is provided a solidstate light-emitting device comprising: a first electrode coupled to afirst charge injecting layer; a second electrode coupled to a secondcharge injecting layer; an emissive layer comprising a semiconductingperovskite material, wherein the emissive layer is provided between thefirst and second charge injecting layers; wherein the semiconductingperovskite material comprises a thin layer of semiconducting perovskitenanoparticles embedded in a matrix or blend of a material that has awider band gap than the semiconducting perovskite nanoparticlesaccording to the second aspect of the present invention.

In a fifth aspect of the present invention, there is provided a solidstate light-emitting device comprising: a first electrode coupled to acharge injecting layer; a second electrode; an emissive layer comprisinga semiconducting perovskite material, wherein the emissive layer isprovided between the charge injecting layer and the second electrode;and wherein the semiconducting perovskite material comprises a thinlayer of semiconducting perovskite nanoparticles embedded in a matrix orblend of a material that has a wider band gap than the semiconductingperovskite nanoparticles according to the second aspect of the presentinvention.

In a sixth aspect of the present invention, there is provided a methodof manufacturing a solid state light-emitting device according to thefourth aspect of the present invention, the method comprising: providinga first electrode on a substrate; depositing a first conducting layerover the first electrode; depositing an emissive semiconductingperovskite layer over the first conducting layer; depositing a secondconducting layer over the emissive semiconducting perovskite layer; anddepositing a second electrode over the second conducting layer; whereinthe semiconducting perovskite layer comprises semiconducting perovskitenanoparticles embedded in a matrix or blend of a material that has awider band gap than the semiconducting perovskite nanoparticles preparedby blending a solution of a semiconducting perovskite material or aprecursor therefor with a solution of a material that has a wider bandgap than the semiconducting perovskite nanoparticles or a precursortherefor, depositing the mixture thus formed on the first conductinglayer followed by removal of the solvent from the mixture thus formed,to give the desired semiconducting perovskite nanoparticles embedded inthe matrix or blend of the material that has a wider band gap than thesemiconducting perovskite nanoparticles.

In a seventh aspect of the present invention, there is provided anelectronic device comprising an LED display, wherein the displaycomprises light-emitting devices according to the fourth or fifth aspectof the invention or prepared according to the method of the sixth aspectof the invention.

In an eighth aspect of the present invention, there is provided the useof an insulating material to fill voids in semiconducting perovskitefilms to prevent non-radiative current losses and reduced efficiency ofthe semiconducting perovskite films when used in solid statelight-emitting devices.

In a ninth aspect of the present invention, there is provided the use ofa semiconducting organic insulating material selected from a polymer anda small organic molecule for the preparation of a matrix comprisingsemiconducting perovskite nanoparticles.

In a tenth aspect of the present invention, there is provided the use ofthe matrix prepared according to the ninth aspect of the presentinvention to facilitate exciton energy transfer to the semiconductingperovskite nanoparticles embedded in the matrix to promoteelectroluminescence.

In an eleventh aspect of the present invention, there is provided theuse of a matrix or blend of an insulating material for the control ofthe wavelengths emitted by a solid state light-emitting device accordingto the third aspect or the fourth aspect of the present invention byvarying the ratio of the insulating material or a precursor therefor tothe semiconducting perovskite material or a precursor therefor in theformation of the emissive layer of the solid state light-emittingdevice.

In an twelfth aspect of the present invention, there is provided the useof a thin layer of semiconducting perovskite nanoparticles embedded in amatrix or blend of a material that has a wider band gap than thesemiconducting perovskite nanoparticles according to the second aspectof the present invention in the preparation of an emissive phosphor.

Preferred embodiments are defined in the appended dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is diagrammatically illustrated, by way of example, in theaccompanying drawings, in which:

FIG. 1a shows a device architecture comprising CH₃NH₃PbBr₃ perovskite,according to an embodiment of the invention;

FIG. 1b shows the chemical structure of PI2525 polyimide precursor(PIP);

FIG. 1c shows an image of a CH₃NH₃PbBr₃/PIP PeLED according to thepresent invention;

FIG. 1d shows the absorption spectra of PIP, CH₃NH₃PbBr₃ and blend film,and electroluminescence spectrum (dashed line) of CH₃NH₃PbBr₃/PIP PeLED;

FIG. 1e shows X-ray diffraction data of films spin-coated on quartzsubstrate under different PIP/perovskite blending ratios;

FIG. 2a shows a graph of combined current density (solid line) vs.voltage and luminance (dashed line) vs. voltage characteristics ofPeLEDs in an embodiment of the invention;

FIG. 2b shows a graph of external quantum efficiency vs. current densitycharacteristics of PeLEDs in an embodiment of the invention;

FIG. 2c shows a graph of external quantum efficiency peak value vs.PIP/CH₃NH₃PbBr₃ weight ratio in an embodiment of the invention;

FIG. 3 shows electroluminescence spectra of F8 LED, perovskite-only andPIP/perovskite blend LEDs, taken at 5V bias;

FIG. 4a shows photoluminescence spectra of samples with perovskitenanocrystals in CBP matrix (with varying weight ratios);

FIG. 4b shows X-ray diffraction patterns of CBP: CH₃NH₃PbBr₃ perovskitethin-films with various CBP:Pe weight ratios. The broadening of the(100) and (200) crystalline peaks indicates the formation of smallernanocrystals.

FIG. 5a shows photoluminescence spectrum of CH₃NH₃PbBr₃ perovskitenanocrystals embedded in a PS matrix (PS:Pe=10:1 weight ratio);

FIG. 5b shows photoluminescence spectrum of CH₃NH₃PbBr₃ perovskitenanocrystals embedded in a PVK matrix (PVK:Pe=3:1 weight ratio);

FIG. 6 shows photoluminance spectrum of alumina nanoparticle/perovskitecomposite films;

FIG. 7 shows absorption spectra of a CH₃NH₃PbBr₃ only film and aCH₃NH₃PbBr₃/alumina blend film; and

FIG. 8 shows photoluminescence spectroscopy data of CH₃NH₃PbBr₂Inanocrystals in an alumina matrix.

DETAILED DESCRIPTION

The method of the first aspect of the present invention provides asolution to the problem of pinhole formation in semiconductingperovskite films. The process of the first aspect of the presentinvention enables the preparation by means of solution deposition ofthin layer of semiconducting perovskite nanoparticles embedded in amatrix or blend of a material that has a wider band gap than thesemiconducting perovskite nanoparticles. The material having a widerband gap than the semiconducting perovskite nanoparticles forms apinhole-free charge-blocking layer, while still allowing, for example,the embedded perovskite crystals to form electrical contact withelectron- and hole-injection layers in one form of light emitting diode.This modified structure reduces non-radiative current losses andimproves quantum efficiency. This simple technique provides analternative route to circumvent film formation problems insemiconducting perovskite optoelectronics, and offers the possibility offlexible and high-performance light-emitting displays.

In the following, references to the present invention refer to allaspects of the present invention unless specifically stated to thecontrary.

The semiconducting perovskite nanoparticles of the present invention arepreferably semiconducting perovskite nanocrystals.

In the context of the present invention, the material in which thesemiconducting perovskite nanoparticles are embedded is a material whichhas a wider band gap than that of the semiconducting perovskitenanoparticles. As a consequence of this, any charges that havepreviously been able to pass through the semiconducting perovskite filmas a result of non-radiative charge leakage through pinholes in the filmare prevented by the band gap barrier thus provided. Preferably, thematerial which has a wider band gap than that of the semiconductingperovskite nanoparticles has a band gap that is greater than 1.5 eV,e.g. greater than 2 eV, greater than 2.5 eV or greater than 2.8 eV.

The material that has a wider band gap than the semiconductingperovskite nanoparticles is suitably an insulating material or asemiconducting material. Preferably it is an insulating materialselected from an insulating polymer, an insulating small organicmolecule and an insulating inorganic material, more preferably aninsulating polymer or an insulating small organic molecule. In oneembodiment, the insulating polymer or the insulating small organicmolecule is a polar polymer or a polar small organic molecule. Theinsulating material may also be a semiconductor.

In one preferred embodiment of the present invention, the insulatingmaterial is a polyimide. The polyimide may be, for example, the polyamicacid of benzophenone tetracarboxylic dianhydride 4,4-oxydianilinem-phenylenediamine polymer (PIP) having the following formula:

In an alternative preferred embodiment of the present invention, theinsulating material is polystyrene.

In yet another embodiment of the present invention, the insulatingmaterial is poly (9-vinylcarbazole), with the following molecularstructure:

In a further embodiment of the present invention, the insulatingmaterial is the small organic compound4,4-bis(N-carbazolyl)-1,1-biphenyl, with the following molecularstructure:

In an alternative embodiment of the present invention, the insulatingmaterial is alumina.

In the present invention, the semiconducting perovskite is preferably asemiconducting perovskite material having the formula AMX₃ wherein A, Mand X can be any suitable balancing combination of catonic and anionicspecies; and more preferably it is an organometal halide perovskitematerial.

In one embodiment of the present invention, the preferred organometalhalide perovskite has an AMX₃ structure, where A is a monovalent cation,M is a divalent cation and X is a halide anion. In AMX₃, the followingare preferred:

(i) M is a divalent metal cation;

(ii) the divalent metal cation M is tin (Sn²⁺) or lead (Pb²⁺);

(iii) the monovalent cation is a primary, secondary or tertiary ammoniumcation [HNR¹R²R³]⁺, wherein each of R¹, R² and R³ may be the same ordifferent and is selected from hydrogen, an unsubstituted or substitutedC₁-C₂₀ alkyl group and an unsubstituted or substituted C₅-C₁₈ arylgroup. Examples of suitable substituents for the alkyl groups are alkoxygroups having from 1 to 20 carbons atoms, hydroxyl groups, mono anddialkylamino groups wherein each alkyl group may be the same ordifferent and has from 1 to 20 carbon atoms, cyano groups, nitro groups,thiol groups, sulphinyl groups, sulphonyl groups and aryl groups havingfrom 5 to 18 carbon atoms. Examples of suitable substituents for thealkyl groups are alkyl groups having from 1 to 20 carbon atoms, alkenyland alkynyl groups each having from 2 to 20 carbon atoms, alkoxy groupshaving from 1 to 20 carbons atoms, haloalkyl groups having from 1 to 20carbon atoms, hydroxyl groups, mono and dialkylamino groups wherein eachalkyl group may be the same or different and has from 1 to 20 carbonatoms, cyano groups, nitro groups, thiol groups, sulphinyl groups andsulphonyl groups;

(iv) the monovalent cation is of the form [R¹R²N—CH═NR³R⁴]⁺:

wherein each of R¹, R², R³ and R⁴ may be the same or different and isselected from hydrogen, an unsubstituted or substituted C₁-C₂₀ alkylgroup and an unsubstituted or substituted C₅-C₁₈ aryl group. Examples ofsuitable substituents for the alkyl groups are alkyl groups having from1 to 20 carbon atoms, alkenyl and alkynyl groups each having from 2 to20 carbon atoms, alkoxy groups having from 1 to 20 carbons atoms,haloalkyl groups having from 1 to 20 carbon atoms, hydroxyl groups, monoand dialkylamino groups wherein each alkyl group may be the same ordifferent and has from 1 to 20 carbon atoms, cyano groups, nitro groups,thiol groups, sulphinyl groups and sulphonyl groups;

(v) the monovalent cation is of the form (R¹R²N)(R³R⁴N)C═NR⁵R⁶:

wherein each of R¹ R², R³, R⁴, R⁵ and R⁶ may be the same or differentand is selected from hydrogen, an unsubstituted or substituted C₁-C₂₀alkyl group and an unsubstituted or substituted C₅-C₁₈ aryl group.Examples of suitable substituents for the alkyl groups are alkyl groupshaving from 1 to 20 carbon atoms, alkenyl and alkynyl groups each havingfrom 2 to 20 carbon atoms, alkoxy groups having from 1 to 20 carbonsatoms, haloalkyl groups having from 1 to 20 carbon atoms, hydroxylgroups, mono and dialkylamino groups wherein each alkyl group may be thesame or different and has from 1 to 20 carbon atoms, cyano groups, nitrogroups, thiol groups, sulphinyl groups and sulphonyl groups;

(vi) the monovalent cation is an alkali metal cation;

(vii) the monovalent cation is caesium (Cs⁺) or rubidium (Rb⁺);

(viii) X is a halide anion selected from chloride, bromide, iodide, andfluoride and, in the AMX₃ structure each halide may be the same ordifferent.

In another embodiment of the present invention, the preferredorganometal halide perovskite has an A_(1-i)B_(i)MX₃ structure, wherein:

-   -   A and B are each a monovalent cation as defined above, where A        and B are different;    -   M is a divalent metal cation as defined above;    -   X is a halide anion as defined above; and    -   i is between 0 and 1.

In a further embodiment of the present invention, the preferredorganometal halide perovskite material has an AMX_(3-k)Y_(k) structure,wherein:

-   -   A is a monovalent cation as defined above;    -   M is a divalent metal cation as defined above;    -   X and Y are each a halide anion as defined above, where X and Y        are different; and    -   k is between 0 and 3.

In a yet further embodiment of the present invention, the preferredorganometal halide perovskite material has an AM_(1-j)N_(j)X₃ structure,wherein:

-   -   A is a monovalent cation as defined above;    -   M and N are each a divalent metal cation as defined above;    -   X is a halide anion as defined above; and    -   j is between 0 and 1.

In a further embodiment of the present invention, the preferredorganometal halide perovskite material has anA_(1-i)B_(i)M_(1-j)N_(j)X_(3-k)Y_(k) structure, wherein:

-   -   A and B are each a monovalent cation as defined above, where A        and B are different;    -   M and N are each a divalent metal cation as defined above;    -   X and Y are each a halide anion as defined above, where X and Y        are different; and    -   where i is between 0 and 1, j is between 0 and 1, and k is        between 0 and 3.

In the present invention, the thickness of the thin layer ofsemiconducting perovskite nanoparticles embedded in the matrix or blendof the material that has a wider band gap than the semiconductingperovskite nanoparticles is preferably ≤500 nm, and more preferably ≤100nm.

In the method according to the first aspect of the present invention, anannealing step may be performed after the step to remove the solvent.The solvent may be removed by any suitable means known to the personskilled in the art, e.g. spin coating.

In the present invention, the ratio by weight of semiconductingperovskite nanoparticles to the material that has a wider band gap thanthe semiconducting perovskite nanoparticles in which the semiconductingperovskite nanoparticles are embedded may for example be from 0.01:1 to20:1, preferably from 0.1:1 to 10:1, more preferably from 1:1 to 5:1,and most preferably from 1:1 to 2:1.

The method of the present invention produces a thin layer ofsemiconducting perovskite nanoparticles embedded in a matrix or blend ofa material that has a wider band gap than the semiconducting perovskitenanoparticles that is particularly well suited for use in devices thatincorporate an emissive semiconducting layer, e.g. the solid statelight-emitting devices according to the fourth and fifth aspects of thepresent inventions.

The applicant has found that by varying the ratio of semiconductingperovskite nanoparticles to the material that has a wider band gap thanthe semiconducting perovskite nanoparticles in which the perovskitenanoparticles are embedded, it is possible to improve the quantumefficiency dramatically, e.g. the applicant has found that in lightemitting diodes prepared according to the invention the external quantumefficiency (EQE) of a device comprising a emissive film according to thepresent invention increased by more than two orders of magnitudecompared to the same device prepared using the same semiconductingperovskite nanoparticles prepared as a film in the absence of thematerial that has a wider band gap than the semiconducting perovskitenanoparticles. It also resulted in a significantly reduced currentdensity required to achieve the same electroluminescence.

This approach can be applied to use of the same thin layer ofsemiconducting perovskite nanoparticles embedded in a matrix or blend ofa material that has a wider band gap than the semiconducting perovskitenanoparticles in semiconducting perovskite light emitting diodes andsolar cells.

The sixth aspect of the present invention provides a method ofmanufacturing of a solid-state light emitting device according to thefourth aspect of the invention.

In one embodiment of the fourth, fifth and sixth aspects of the presentinvention, the first electrode is an anode and is formed by thedeposition of a transparent conductive material, preferably selectedfrom indium tin oxide (ITO), fluorine doped tin oxide (FTO), indium zincoxide, graphene, carbon nanotubes, and a metal with a thickness of lessthan 20 nm, and more preferably indium tin oxide (ITO). In the fourthaspect and sixth aspect of the present invention, the first electrode asanode is deposited on a substrate which is usually formed of atransparent material, preferably glass.

In another embodiment of the fourth, fifth and sixth aspects of thepresent invention, the emissive semiconducting perovskite layer iscomposed of semiconducting perovskite nanocrystals embedded in a matrixor a blend of an insulating material selected from an insulatingpolymer, an insulating small organic molecule and an insulatinginorganic compound.

In a further embodiment of the fourth, fifth and sixth aspects of thepresent invention, the emissive layer has a thickness of ≤500 nm,preferably ≤100 nm.

In a yet further embodiment of the fourth, fifth and sixth aspects ofthe present invention, a thin insulating layer is deposited betweeneither or both of the charge injecting layers and the light-emittinglayer. Preferably, the insulating layer is formed of an oxide ornitride. More preferably the insulating layer is selected from the groupconsisting of aluminium oxide, silicon dioxide, silicon nitride, zincoxide, nickel oxide or magnesium oxide. The insulating layer may bedeposited by any suitable means, e.g. atomic layer deposition, ALD.

For example, the transparent conductive electrode material may be coatedwith a thin layer (preferably <30 nm thick) of an insulating layerselected from the group consisting of aluminium oxide, silicon dioxide,zinc oxide, magnesium oxide, nickel oxide and silicon nitride. The thinscaffold or interlayer may improve the luminescence efficiency of thedevice, by preventing quenching at the interface between the chargeinjection layer and the emissive perovskite layer, or by preventingelectrons or holes from escaping out of the semiconducting perovskitelayer.

In a further embodiment of the fourth, fifth and sixth aspects of thepresent invention, at least one of the first conducting layer and thesecond conducting layer is formed of a semiconductor material. One ofthe conducting layers (charge injecting layers) may be a hole injectingorganic semiconducting material and may be, for example, selected fromthe group consisting of PEDOT:PSS, PANI (polyaniline), polypyrole,optionally substituted, and doped poly(ethylene dioxythiophene) (PEDT).One of the conducting layers (charge injecting layers) may be anelectron injecting organic semiconducting material and may be, forexample, selected from the group consisting of poly(fluorene)s such asF8 and F8-PFB AB copolymer (95:5 F8:TFB).

In one preferred embodiment of the fourth, fifth and sixth aspects ofthe present invention, a thin layer of <30 nm of a material selectedfrom molybdenum trioxide and tungsten trioxide is deposited between:

the transparent conductive electrode and the semiconducting perovskitelayer,

between a charge injecting layer and a conductive electrode,

between the transparent conductive electrode and a charge injectinglayer,

between the semiconducting perovskite layer and a charge injectinglayer, or

between the semiconducting perovskite layer and a conductive electrode.

This further thin layer may increase the charge injection efficiencybetween layers of the light-emitting device.

The deposition of each of the layers in the method of the sixth aspectof the invention may be performed using one or more of the followingdeposition techniques: vacuum thermal evaporation, spin coating,direct-write printing, inkjet printing, lithographic patterning, andsolution deposition.

The seventh aspect of the present invention provides an electronicdevice comprising an LED display, wherein the display compriseslight-emitting devices as described above. The electronic device may forexample be a consumer electronic device, a mobile device such assmartphone or tablet PC, or a display screen/panel.

The eighth aspect of the present invention is directed to the use of aninsulating material to fill voids in semiconducting perovskite films toprevent non-radiative current losses and reduced efficiency of thesemiconducting perovskite films when used in solid state light-emittingdevices. In particular, the insulating material is preferably aninsulating polymer, an insulating small organic molecule and aninsulating inorganic material as described and exemplified above.

The ninth aspect of the present invention provides the use of asemiconducting organic insulating material selected from a polymer and asmall organic molecule for the preparation of a matrix comprisingsemiconducting perovskite nanoparticles. Preferred insulating polymersand insulating small organic molecules are as described and exemplifiedabove. The matrix thus prepared can be used to facilitate exciton energytransfer to the semiconducting perovskite nanoparticles embedded in thematrix to promote electroluminescence in accordance with the tenthaspect of the present invention.

The eleventh aspect of the present invention provides the use of aninsulating material for the control of the wavelengths emitted by asolid state light-emitting device by varying the ratio of the insulatingmaterial or a precursor therefor to the semiconducting perovskitematerial or a precursor therefor in the formation of the emissive layerof the solid state light-emitting device. The insulating material ispreferably an insulating polymer, an insulating small organic moleculeand an insulating inorganic material as described and exemplified above.The solid state light-emitting device can be any of the solid statelight-emitting devices described and exemplified above according to thefourth and fifth aspects of the present invention or prepared accordingto the sixth aspect of the present invention. The ratio of insulatingmaterial or a precursor therefor to the semiconducting perovskitematerial or a precursor therefor in the formation of the emissive layerof the solid state light-emitting device may typically be from 0.01:1 to20:1, preferably 0.1:1 to 10:1, more preferably from 1:1 to 5:1, andmost preferably from 1:1 to 2:1.

The twelfth aspect of the present invention provides the use of a thinlayer of semiconducting perovskite nanoparticles embedded in a matrix orblend of a material that has a wider band gap than the semiconductingperovskite nanoparticles in the preparation of an emissive phosphor. Thethin layer of semiconducting perovskite nanoparticles embedded in amatrix or blend of a material that has a wider band gap than thesemiconducting perovskite nanoparticles is as described and exemplifiedabove in the second aspect of the invention. The material that has awider band gap than the semiconducting perovskite nanoparticles ispreferably selected from an insulating organic material selected from aninsulating polymer and an insulating small organic molecule, and a blendof alumina nanoparticles, and more preferably a blend of aluminananoparticles. The ratio by weight of semiconducting perovskitenanoparticles:alumina nanoparticles in this preferred embodiment istypically from 1:10 to 10:1, preferably from 2:1 to 6:1, e.g. 6:1, 4:1and 2:1.

The applicant has found that blending semiconducting perovskiteprecursors with a material that has a wider band gap than thesemiconducting perovskite nanoparticles, preferably alumina, anddepositing them into a film increases the photoluminescence efficiencyof the perovskite. Alumina is transparent (in the visible), and does notabsorb semiconducting perovskite emission. It can be processed fromsolution, similar to semiconducting perovskite.

EXAMPLES

FIG. 1a shows device architecture 10 comprising a thin layer ofsemiconducting perovskite nanoparticles embedded in a polyimideprecursor (PIP), according to an embodiment of the invention. Theperovskite light-emitting diode (PeLED) 10 is fabricated on atransparent substrate 12, which may be a glass substrate. Generallyspeaking, the PeLED illustrated here comprises a first electrode 14coupled to a first charge injecting layer 16, a second electrode 22coupled to a second charge injecting layer 20, and an active layer 18formed of organometal halide perovskite nanoparticles embedded in PIP(e.g. a CH₃NH₃PbBr₃: PIP layer). The active layer 18 is sandwichedbetween the first and second charge injecting layers 16, 20. One or bothof the charge injecting layers 16, 20 may be formed of a semiconductormaterial.

Preferably, the first electrode 14 is formed of a transparent,conductive material, such as indium tin oxide (ITO). One or both of thefirst and second charge injecting layers 16, 20 may actively injectcharge into the active perovskite layer 18, or one or both of layers 16,20 may simply act as a charge transport and charge blocking layer. Thesecond electrode 22 is formed of any conductive material such as, butnot limited to MoO₃/Ag, Ca/Ag, Au etc.

For illustrative purposes, the visible light emitting PeLED device 24 isfabricated using an ITO/PEDOT:PSS/CH₃NH₃PbBr₃:PIP/F8/Ca/Ag architecture.Thus, here, the first charge injecting layer 16 is formed from aconductive polymer material, such aspoly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS), andthe second charge injecting layer 20 is formed frompoly(9,9′-dioctylfluorene) (F8). The PIP polymer generally has a largerbandgap than the semiconducting perovskite nanoparticles. In thisstructure, the injected charges are confined within the active layer 18by providing the charge injecting layers 16, 20 from a material that hasa large bandgap relative to the bandgap of the perovskite layer.Additionally, confinement of the injected charges (holes and electrons)within the active (emissive) perovskite layer may be improved further byforming the PeLED with a thin active perovskite layer, e.g. having athickness of less than 100 nm (or <60 nm, or even <20 nm).

A large-bandgap perovskite, CH₃NH₃PbBr₃, was used as a green emitter.The perovskite precursors are mixed with a commercial aromatic polyimideprecursor (PIP). FIG. 1b shows the chemical structure of a PI2525polyimide precursor (PIP). This polymer was chosen for its polarfunctional groups, which are likely to associate well with the hybridperovskite. The perovskite nanocrystal and PIP polymer composite formsupon spin-coating and mild annealing, although the polymer remains inits unreacted precursor form under these processing conditions. Theperovskite crystals form well, even in the presence of a polymer matrix,as confirmed by x-ray diffraction studies. FIG. 1e shows X-raydiffraction data of films spin-coated on quartz substrate underdifferent PIP/perovskite blending ratios. The crystal size ofperovskite-only and PIP/perovskite 1/10 w/w films cannot be determinedbecause of instrumental broadening. The estimated crystal size forPIP/perovskite 1/2 w/w is 70 nm.

Here, ITO is the anode (electrode 14) and is coupled to the first chargeinjecting layer 16, which may be formed from PEDOT:PSS and is used toinject holes into the active perovskite layer 18. The second electrode22 is the cathode and may be formed from silver/calcium.

The second electrode 22 is coupled to the second charge injecting layer20, which may be formed from a poly(9,9′-dioctylfluorene) (F8) polymersemiconductor material. The F8 material serves as a spacer layer forelectron transport and to prevent emission quenching near the interfacewith the second electrode 22 (rather than actively injecting charge intothe perovskite layer). The F8 material serves as theelectron-transporting layer 20, blocks injected holes from leaving thedevice 10, and also serves as a spacer layer to prevent luminescencequenching at the metal electrode 22. In addition, the semiconducting F8layer 20 forms a good conformal coating and prevents the top electrode22 from shorting through the thin (<50 nm) perovskite layer 18. Thelow-workfunction calcium electrode 22 provides ohmic electron injection,while the high-workfunction PEDOT:PSS acts as the hole-injection layer16.

FIG. 1d shows the absorption spectra of PIP, CH₃NH₃PbBr₃ and blendfilms, and the electroluminescence spectrum (dashed line) of theCH₃NH₃PbBr₃/PIP PeLED shown in FIG. 1a . As shown in FIG. 1d , a stronggreen electroluminescence was observed at 529 nm, with a narrow fullwidth at half maximum (FWHM) of 19 nm. The PIP polymers absorb only inthe UV and do not interfere with the green emission from the perovskite.FIG. 1c shows an image of CH₃NH₃PbBr₃/PIP PeLED, in an embodiment of theinvention. As shown in FIG. 1c , electroluminescence is uniform acrossthe entire device pixel with no signs of spotting, demonstrating thatthe perovskite nano-crystals are uniformly distributed across thespin-coated film.

To determine the effectiveness of the perovskite:PIP polymer compositeas an emissive active layer, light-emitting diodes with a range of PIPto perovskite weight ratios were fabricated. FIG. 2a shows the combinedluminance and current density versus voltage characteristics of deviceswith PIP:perovskite ratios of 1:10, 1:2 and 1:1 as well as a controldevice with no PIP added. FIG. 2b shows the corresponding externalquantum efficiency (EQE) of the respective devices. The devices show aclear trend of increased quantum efficiency with the addition of PIP. Inparticular, the EQE increased by more than 2 orders of magnitude, from0.010% in devices without PIP, to 1.2% in devices with a 1:1 PIP toperovskite ratio. This corresponds to an internal quantum efficiency(IQE) of 5.4%, assuming a Lambertian emission profile. This dramaticenhancement in efficiency is mostly led by a decrease in current densityrequired for electroluminescence. For instance, in the 1:2 ratio device,a current density of 3.1 mA cm⁻² is required to produce a luminance of200 cd m⁻², but much larger current densities of 580 mA cm⁻² and 57 mAcm⁻² are required for the perovskite-only and the 1:10 ratio devicesrespectively to achieve the same luminance.

The PIP and perovskite composite devices were investigated over a widerrange of mixing ratios—the device performances are shown in FIG. 2c .The best EQE of ˜1% is achieved between a 1:2 and 1:1 PIP to perovskiteratio. This represents a 10-fold enhancement of device efficiency overthose reported in earlier studies. The device efficiency drops at higherratios, and some device pixels fail to work at these ratios, suggestingthat the emissive layer has become too resistive with such high polymercontent.

Through scanning electron microscope studies of films of differentratios, we have been able to deduce that the perovskite nano-crystalsform within a matrix of dielectric PIP in the blend films, and theinsulating PIP polymer serves to block the electrical shunting paths.The thickness of the PIP polymer was determined to be 3 nm and 30 nm inthe 1:10 and 1:2 ratio PIP:perovskite films, respectively. The thicker1:2 ratio film provides superior PIP coverage and therefore leads tominimal current losses and a significantly enhanced device efficiency.The polymer coverage in the 1:10 ratio films is thinner and possiblyincomplete in some areas, and therefore provides less protection againstcurrent losses. Since the perovskite and PIP composite devices emitefficiently and uniformly up to a 1:1 blend ratio, it is reasonable toassume that the perovskite nano-crystals extend across the thickness ofthese films, forming electrical contact with both the PEDOT:PSS and theF8 layers.

FIG. 3 shows electroluminescence spectra of F8 LED, perovskite-only andPIP/perovskite blend LEDs, taken at 5V bias. The charges that meet andrecombine in the perovskite-covered regions produce green lightemission. However, in the spaces between the perovskite crystals, thecharges are either lost at the exposed F8/PEDOT:PSS interface, or areblocked by the PIP dielectric polymer in the composite films.Recombination of charges in F8 could lead to F8 electroluminescence.Indeed, in the perovskite-only device, an F8 contribution toelectroluminescence could be observed between 400 nm and 500 nm, asshown in FIG. 3. This F8 electroluminescence is completely shut off inthe PIP-containing devices and a clean perovskite emission is obtained,further demonstrating that the PIP is remarkably effective in blockingthe shunting paths at the F8/PEDOT:PSS interface.

Size Tunablility of the Perovskite Nanocrystals Formed in an OrganicMatrix

In embodiments, it is possible to tune the properties of the perovskitelayer in a light-emitting diode or an emissive phosphor by altering thesize of the nanocrystals formed in the organic matrix. For example, itis desirable to tune the emission wavelength (colour) of emissivephosphors.

Scanning electron microscopy (SEM) images of CBP:perovskite samples(CH₃NH₃PbBr₃ perovskite nanocrystals in4,4-Bis(N-carbazolyl)-1,1-biphenyl (CBP) matrix) mixed at weight ratiosranging from 1:1 to 15:1 (CBP:perovskite), show that the relativelybright regions in the images correspond to CH₃NH₃PbBr₃ perovskite (Pe),while the darker regions correspond to the CBP matrix. The contrast ofthe two materials is due to the difference in their conductivities. Whenthe weight concentration of perovskite is 50% (CBP:Pe=1:1), theperovskite material forms a connected network, enclosing isolatedislands of CBP. The mean width of the perovskite clusters is about 280nm. In contrast, for a sample having CBP:Pe=15:1 (i.e. a lower amount ofperovskite material embedded within the matrix), the average size of theperovskite nanocrystal domains is approximately 50 nm. Each perovskitenanocrystal domain is formed by many smaller nanocrystals.

FIG. 4a shows photoluminescence spectra of samples with perovskitenanocrystals in a CBP matrix (with varying weight ratios). Inparticular, the spectra show the photoluminescence (PL) spectra ofCH₃NH₃PbBr₃ perovskite nanocrystals embedded in a4,4-Bis(N-carbazolyl)-1,1-biphenyl (CBP) matrix. The samples wereoptically excited by a 407 nm laser during the measurement. The PLspectrum of a bulk (pristine) perovskite thin-film is shown as the redcurve. The peak of the emission shifts to shorter wavelength as theconcentration (and size) of perovskite in CBP matrix decreases.

FIG. 4b shows X-ray diffraction patterns of CBP: CH₃NH₃PbBr₃ perovskitethin-films with various CBP:Pe weight ratios. The broadening of the(100) and (200) crystalline peaks indicates the formation of smallernanocrystals. The X-ray diffraction results shown in FIG. 4b , revealthat the average size of the perovskite nanocrystal is about 76 nm forthe sample having CBP:Pe=1:1, and about 10.5 nm for the sample withCBP:Pe=15.1. Thus, experiments show that as the concentration ofperovskite decreases, the perovskite materials form nanocrystals withdecreasing sizes.

Perovskite nanocrystals formed in other polymer materials have also beeninvestigated, to determine further methods of tuning thephotoluminescence properties of the perovskite nanocrystals. Forexample, CH₃NH₃PbBr₃ perovskite nanocrystals were embedded in anpolystyrene (PS) matrix (PS:Pe=10:1 weight ratio). Scanning electronmicroscope (SEM) images reveal that the average size of the nanocrystaldomains was approximately 180 nm. FIG. 5a shows the photoluminescencespectrum of CH₃NH₃PbBr₃ perovskite nanocrystals embedded in a PS matrix(PS:Pe=10:1 weight ratio). Perovskite nanocrystals were also formed in aconjugated polymer matrix, e.g. CH₃NH₃PbBr₃ perovskite nanocrystals in apoly(9-vinylcarbazole) (PVK) matrix. FIG. 5b shows the photoluminescencespectrum of CH₃NH₃PbBr₃ perovskite nanocrystals embedded in a PVK matrix(PVK:Pe=3:1 weight ratio). Thus, it is clear that the matrix in whichthe perovskite nanocrystals are embedded can affect thephotoluminescence.

For FIGS. 5a and 5b , the CH₃NH₃PbBr₃ perovskite precursor solution wasprepared by mixing CH₃NH₃Br and PbBr₂ at a molar ratio of 3:1 inanhydrous N,N-dimethylformamide (DMF) to give a weight concentration of20%. The solution was further diluted in DMF to give a concentration of10 mg/ml. The solution was stirred for 24 hours before furtherprocessing.

4,4-Bis(N-carbazolyl)-1,1-biphenyl (CBP) was dissolved in DMF to aconcentration of 10 mg/ml. Polystyrene (PS) was dissolved in DMF to aconcentration of 20 mg/ml. Poly(9-vinylcarbazole) (PVK) was dissolved inDMF to a concentration of 10 mg/ml. The solutions were stirred for 24 hrbefore further processing.

The CH₃NH₃PbBr₃ perovskite precursor solution and the matrix materialsolutions were mixed at various volume ratios to achieve the weightratios described in the examples above. The mixtures were stirred under70° C. for 2 hours and were spun-cast onto pre-cleaned quartz substratesin a N₂ purged glovebox to obtain a film thickness of about 50 nm. Thethin-films were then annealed at 90° C. for 5 min to accelerate solventevaporation and crystallisation.

Perovskite Nanoparticles for use in an Emissive Phosphor

We have described above how the properties of perovskite nanoparticlefilms can be tuned to alter the emission wavelengths, and how the tuningtechniques can be used to provide tunable emissive phosphors. Thepossibility of using alumina nanoparticle/perovskite as an emissivephosphor has been investigated. In embodiments, perovskite precursorsare blended with alumina, and deposited into a film which increases theluminescence efficiency of the perovskite. Alumina is transparent (inthe visible spectrum), and does not absorb perovskite emission. Aluminacan be processed from solution, similar to perovskite, which maysimplify the production of an emissive phosphor.

The CH₃NH₃PbBr₃ precursor was prepared by mixing CH₃NH₃Br and PbBr₂ inDMF at a molar ratio of 3:1, at concentrations of 5 wt %, 2.5 wt % and1.7 wt %. Alumina nanoparticle dispersion in isopropanol (20 wt %, <50nm particle size) was diluted using DMF to achieve differentconcentrations. The CH₃NH₃PbBr₃/alumina blending solution was made byblending perovskite solutions with the alumina dispersions.Alumina/perovskite composite films were spin-coated on oxygen plasmacleaned quartz spectrosil in a nitrogen glove box at 3000 rpm for 30 s,before being annealed at 60° C. for 1 minute. FIG. 6 shows thephotoluminescence spectra of alumina nanoparticle/perovskite compositefilms with a range of alumina nanoparticle to perovskite weight ratios.It is clear that the peak photoluminescence may be tuned by altering theratio of alumina nanoparticle to perovskite ratio, and in particular,the peak can be shifted towards shorter wavelengths by increasing thealumina nanoparticle concentration. FIG. 7 shows the absorption spectraof CH₃NH₃PbBr₃ only film and CH₃NH₃PbBr₃/alumina in a 1:4 weight ratioblend in more detail.

Alternatively, perovskite nanocrystals may be formed using CH₃NH₃PbBr₂Iparticles in an alumina matrix. In this embodiment, the perovskiteprecursor is prepared by mixing 5 wt % CH₃NH₃PbBr₃ precursor solution inDMF and 5 wt % CH₃NH₃PbI₃ precursor solution in DMF (3:1 molar ratiobetween CH₃NH₃I and PbI₂) at a volume ratio of 2:1. FIG. 8 showsphotoluminescence spectroscopy data of such CH₃NH₃PbBr₂I nanocrystals inan alumina matrix. The peak photoluminescence is shifted to longerwavelengths relative to that of the CH₃NH₃PbBr₃ nanocrystals (see FIG.6). Thus, further tunability is possible by using different perovskitematerials.

Materials and Fabrication Techniques

When the perovskite materials described above are used in themanufacture of organic optoelectronic devices, they are preferablydissolved to form solutions. The solutions may be used in solutionprocessing techniques to form the active/emissive layer of such devices.The electrodes of the PeLEDs may be deposited by thermal evaporation.The emissive layer, the hole injecting layer and/or interlayer(s) may bedeposited by solution processing, e.g. spin coating. Preferred devicesof the invention are also encapsulated to avoid ingress of moisture andoxygen. Conventional encapsulation techniques may be used.

The PeLED substrate may be formed from any material conventionally usedin the art such as glass or plastic. Optionally the substrate ispre-treated to improve adhesion thereto. Preferably the substrate istransparent. Preferably the substrate also has good barrier propertiesto prevent ingress of moisture or oxygen into the device.

The anode may comprise any material with a workfunction suitable forinjection of holes into the light emitting layer. Preferably the anodeis transparent. Representative examples of materials for use as atransparent anode include indium tin oxide (ITO) and indium zinc oxide(IZO). If the anode is not required to be transparent (e.g. if thecathode is transparent or it is a bottom emitting device) then opaqueconducting materials such as opaque metals may be used as the anode.

The anode may comprise a single layer or may comprise more than onelayer. For example, the anode may comprise a first anode layer and anauxiliary conductive layer between the anode and the hole injectionlayer such as a layer of organic conductive material between the anodeand the hole injection layer.

The anode may be deposited on the substrate by thermal evaporation. Theanode may be between 10 nm to 200 nm thick.

The hole injection layer may be deposited by a solution-based processingmethod. Any conventional solution-based processing method may be used.Representative examples of solution-based processing methods includespin coating, dip coating, slot die coating, doctor blade coating andink-jet printing. In embodiments, spin coating is the preferreddeposition method. The parameters used for spin coating the holeinjection layer such as spin coating speed, acceleration and time areselected on the basis of the target thickness for the layer. Afterdeposition, the hole injection layer is preferably annealed by heating,e.g. at 150 to 200° C. for 5 to 30 minutes in air. The thickness of thehole injection layer may be 15 to 100 nm, or between 30 to 50 nm.

As mentioned above, the devices may incorporate a thin insulating layerformed between either or both of the electrodes and the light-emittinglayer. The insulating layer is preferably formed of an oxide or nitride,and more preferably the insulating layer is selected from the groupconsisting of aluminium oxide, silicon dioxide, silicon nitride, zincoxide, nickel oxide or magnesium oxide. The interlayer can play animportant role in improving the device efficiency and the lifetime ofLEDs (because exciton quenching at the interface between the chargeinjecting layer(s) and the emissive layer is prevented or minimised).Preferably, the insulating layer is deposited by atomic layerdeposition, ALD.

The light-emitting (emissive) layer may be prepared by depositing asolution as hereinbefore defined on the charge injecting layer or, whenpresent, the interlayer. Any conventional solution-based processingmethod may be used. Representative examples of solution-based processingmethods include spin coating, dip coating, slot die coating, doctorblade coating and ink-jet printing. In embodiments, the depositing is byspin coating. The parameters used for spin coating the light emittinglayer such as spin coating speed, acceleration and time are selected onthe basis of the target thickness for the light emitting layer. Afterdepositing, the light emitting layer is preferably dried, e.g. at100-150° C. in a glove box. The thickness of the light emitting layermay be between 50 to 350 nm or between 75 to 150 nm.

The cathode may comprise any material having a workfunction allowinginjection of electrons into the light-emitting layer. In order toprovide efficient injection of electrons into the device, the cathodepreferably has a workfunction of less than 3.5 eV, more preferably lessthan 3.2 eV, most preferably less than 3 eV. Work functions of metalscan be found in, for example, Michaelson, J. Appl. Phys. 48(11), 4729,1977. The cathode may consist of a single material such as a layer ofgold. Alternatively, it may comprise a plurality of metals, for examplea bilayer or trilayer of a low workfunction material and a highworkfunction material such as calcium and aluminium as disclosed in WO98/10621; elemental barium as disclosed in WO 98/57381, Appl. Phys.Lett. 2002, 8 1(4), 634 and WO 02/84759; or a thin layer of metalcompound, in particular an oxide or fluoride of an alkali or alkaliearth metal, to assist electron injection, for example lithium fluorideas disclosed in WO 00/48258; barium fluoride as disclosed in Appl. Phys.Lett. 2001, 79(5), 2001; and barium oxide.

The cathode may be opaque or transparent. Transparent cathodes areparticularly advantageous for active matrix devices because emissionthrough a transparent anode in such devices is at least partiallyblocked by drive circuitry located underneath the emissive pixels. Atransparent cathode comprises a layer of an electron injecting materialthat is sufficiently thin to be transparent. Typically, the lateralconductivity of this layer will be low as a result of its thinness. Inthis case, a layer of electron injecting material is used in combinationwith a thicker layer of transparent conducting material such as indiumtin oxide.

The cathode may be deposited by thermal evaporation. The cathode may be100 to 400 nm thick, or between 200 to 350 nm thick.

Suitable encapsulants include a sheet of glass, films having suitablebarrier properties such as silicon dioxide, silicon monoxide, siliconnitride or alternating stacks of polymer and dielectric as disclosed in,for example, WO 01/81649 or an airtight container as disclosed in, forexample, WO 01/19142. In the case of a transparent cathode device, atransparent encapsulating layer such as silicon monoxide or silicondioxide may be deposited to micron levels of thickness, although in onepreferred embodiment the thickness of such a layer is in the range of20-300 nm. A material for absorption of any atmospheric moisture and/oroxygen that may permeate through the substrate or encapsulant mayoptionally be disposed between the substrate and the encapsulant.

The polyimide precursor (product number PI2525) was diluted into aconcentration of 20 mg/ml in N-methyl-2-pyrrolidone before use.

CH₃NH₃PbBr₃ Perovskite Synthesis. Methylammonium bromide (CH₃NH₃Br) wasprepared by adding 33 wt % methylamine solution in ethanol (24 mL) and48 wt % hydrobromic acid in water (8.5 mL) to 100 mL of absoluteethanol. The reaction mixture was stirred at room temperature. Thesolvent was removed by rotary evaporation. The obtained white crystalswere washed with anhydrous diethyl ether and recrystallized in ethanol.The perovskite precursor solution was prepared by mixing CH₃NH₃Br andPbBr₃ in a 3:2 molar ratio in anhydrous N,N-dimethylformamide to give aconcentration of 5 wt %.

PeLED fabrication. Blend solutions were prepared by mixing 5 wt %CH₃NH₃PbBr₃ solutions and diluted PIP solutions to give different weightratios. These blend solutions were stirred under room temperature for 2hours before use.

ITO-coated glass substrate was cleaned successively with acetone andisopropanol for 15 minutes, followed by 10 minutes oxygen plasmatreatment. PEDOT:PSS (Clevios P VP Al 4083) was spin coated onto thesubstrate at 6000 rpm for 30 seconds and annealed at 140 degree for 30minutes in a nitrogen glovebox. The blend solutions were spin coatedonto PEDOT:PSS at 3000 rpm for 30 seconds and annealed at 60 degree for1 minute. A solution of F8 in chlorobenzene (10 mg mL⁻¹) was spin coatedonto the perovskite layer at 3000 rpm for 30 seconds to give a 50 nmfilm. We note that PIP is insoluble in chlorobenzene and is unaffectedby the deposition of the F8 layer. Ca (20 nm) and Ag (100 nm) weresuccessively deposited by vacuum thermal evaporation at vacuum betterthan 3×10⁻⁶ mbar. Devices were tested in air without encapsulation.

PeLED Characterization. Current vs. voltage characteristics weremeasured using a Keithley 2400 Source Measure Unit (SMU). Photon fluxwas measured simultaneously using a calibrated silicon photodiodecentred over the light-emitting pixel. Luminance in cd m⁻² wascalculated based on the emission spectrum of the PeLED, and on the knownspectral response of the silicon photodiode. External quantum efficiencywas calculated, assuming a Lambertian emission profile.Electroluminescence spectra were measured using a Labsphere CDS-610spectrometer.

Summary

It is shown that luminescent perovskite nanocrystals can be embedded ina pinhole-free matrix of dielectric polymer to give superiorlight-emitting diode performance. This technique is completely solutionprocessed and remarkably simple, and could potentially be extended toperovskite solar cells to solve current leakage problems. Thisincorporation of a polymer matrix further gives the device an addedadvantage of flexibility for an otherwise brittle perovskite material.

It is certainly interesting and perhaps surprising that a crystallineperovskite material can form and associate so well with a polymericmixture without the need for any complex surface modification, and stillretain its remarkable optoelectronic properties. This is possiblygranted by the unique inorganic-organic hybrid nature of the organometalhalide perovskites. It is expected that this extraordinary compatibilityof perovskites with organic materials, coupled with their respectablelight-emitting and optoelectronic performance will make them valuablefor a range of flexible displays and lighting applications.

No doubt many other effective alternatives within the scope of thepresently claimed invention will occur to the skilled person. It will beunderstood that the invention is not limited to the describedembodiments and encompasses modifications apparent to those skilled inthe art lying within the spirit and scope of the claims appended hereto.

The invention claimed is:
 1. An emissive phosphor comprising a layer ofsemiconducting perovskite nanoparticles embedded in a matrix or blend ofan insulating material, wherein the insulating material comprises aninsulating polymer or an insulating organic molecule, and wherein thesemiconducting perovskite comprises perovskite with an AMX₃ structure,where A is a monovalent cation, M is a divalent cation and X is a halideanion, wherein the monovalent cation A is: of the form[R¹R²N═CH—NR³R⁴]⁺:

wherein each of R¹, R², R³ and R⁴ may be the same or different and isselected from hydrogen, an unsubstituted or substituted C₁-C₂₀ alkylgroup, and an unsubstituted or substituted Cs-Cis aryl group; and/or ofthe form (R¹R²N)(R³R⁴)N═NR⁵R⁶:

wherein each of R¹ R², R³, R⁴, R⁵ and R⁶ may be the same or differentand is selected from hydrogen, an unsubstituted or substituted C₁-C₂₀alkyl group, and an unsubstituted or substituted C₅-C₁₈ aryl group;and/or an alkali metal cation.
 2. The emissive phosphor of claim 1,wherein the insulating material is an insulating polymer.
 3. Theemissive phosphor of claim 2, wherein the insulating polymer is a polarpolymer.
 4. The emissive phosphor of claim 1, wherein the insulatingmaterial is an insulating organic molecule.
 5. The emissive phosphor ofclaim 4, wherein the insulating organic molecule is a polar organicmolecule.
 6. The emissive phosphor of claim 1, wherein the insulatingmaterial is a polyimide, including a polyamic acid of benzophenonetetracarboxylic dianhydride 4,4-oxydianiline m-phenylenediamine polymer(PIP) having the following formula:

is polystyrene; or is poly (9-vinylcarbazole), of the form:

is the organic compound 4,4-bis(N-carbazolyl)-1,1-biphenyl, of the form:


7. The emissive phosphor of claim 1, wherein the thickness of the layerof semiconducting perovskite nanoparticles embedded in the matrix orblend of the insulating material is ≤500 nm.
 8. The emissive phosphor ofclaim 1, wherein X is: a halide anion selected from chloride, bromide,iodide, and fluoride and, in the AMX₃ structure each halide may be thesame or different.
 9. The emissive phosphor of claim 1, wherein theperovskite material has an A′_(1-i)B′_(i)MX₃ structure, wherein: A′ andB′ are each a monovalent cation having the same definition as A in claim1, where A′ and B′ are different; and, i is between 0 and 1; or whereinthe perovskite material has an AMX′_(3-k)Y′_(k) structure, wherein: X′and Y′ are each a halide anion having the same definition as X in claim1, where X‘ and Y’ are different; and k is between 0 and 3; or whereinthe perovskite material has an AM′_(1-j)N′_(j)X₃ structure, wherein: M′and N′ are each a divalent metal cation having the same definition as Min claim 1, where M′ and N′ are different; and j is between 0 and 1; orwherein the perovskite material has anA″_(1-i)B″_(i)M″_(1-i)N″_(i)X″_(3-k)Y″_(k) structure, wherein: A″ and B″are each a monovalent cation having the same definition as A in claim 1,where A″ and B″ are different; M″ and N″ are each a divalent metalcation having the same definition as M in claim 1, where M″ and N″ aredifferent; X″ and Y″ are each a halide anion having the same definitionas X in claim 1, where X″ and Y″ are different; and where i is between 0and 1, j is between 0 and 1, and k is between 0 and
 3. 10. The emissivephosphor of claim 1, wherein the semiconducting perovskite nanoparticlesare semiconducting perovskite nanocrystals.
 11. The emissive phosphor ofclaim 1, wherein the ratio by weight of semiconducting perovskitenanoparticles: the insulating material is from 0.01:1 to 20:1.
 12. Amethod to select wavelengths emitted by the emissive phosphor of claim1, said method comprising: selecting a ratio of semiconductingperovskite nanoparticles to the insulating material, and embedding thesemiconducting perovskite nanoparticles in a matrix or blend of theinsulating material according to the ratio to provide an emissivephosphor having said selection of said wavelengths; and/or selecting asemiconducting perovskite nanoparticle material or a precursor thereforand/or selecting the insulating material, and embedding thesemiconducting perovskite nanoparticle material or precursor therefor inthe insulating material, to provide an emissive phosphor having saidselection of said wavelengths.
 13. A method to select wavelengthsemitted by the emissive phosphor of claim 1, said method comprisingvarying the size of the perovskite nanoparticles, and embedding theperovskite nanoparticles with the varied size in a matrix or blend ofthe insulating material, to provide an emissive phosphor having saidselection of said wavelengths.
 14. The emissive phosphor of claim 1,wherein the ratio by weight of semiconducting perovskite nanoparticles:the insulating material is from 0.01:1 to 2:1.
 15. The emissive phosphorof claim 1, wherein the ratio by weight of semiconducting perovskitenanoparticles: the insulating material is from 0.1:1 to 10:1.
 16. Theemissive phosphor of claim 1, wherein the ratio by weight ofsemiconducting perovskite nanoparticles: the insulating material is from1:1 to 5:1.
 17. The emissive phosphor of claim 1, wherein the ratio byweight of semiconducting perovskite nanoparticles: the insulatingmaterial is from 1:1 to 2:1.
 18. The emissive phosphor of claim 1,wherein the alkali metal cation is caesium (Cs⁺) or rubidium (Rb⁺).